/* * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at * http://creativecommons.org/licenses/publicdomain */ package java.util.concurrent; /** * A {@link CompletionService} that uses a supplied {@link Executor} * to execute tasks. This class arranges that submitted tasks are, * upon completion, placed on a queue accessible using take. * The class is lightweight enough to be suitable for transient use * when processing groups of tasks. * *

* * Usage Examples. * * Suppose you have a set of solvers for a certain problem, each * returning a value of some type Result, and would like to * run them concurrently, processing the results of each of them that * return a non-null value, in some method use(Result r). You * could write this as: * *

 *   void solve(Executor e,
 *              Collection<Callable<Result>> solvers)
 *     throws InterruptedException, ExecutionException {
 *       CompletionService<Result> ecs
 *           = new ExecutorCompletionService<Result>(e);
 *       for (Callable<Result> s : solvers)
 *           ecs.submit(s);
 *       int n = solvers.size();
 *       for (int i = 0; i < n; ++i) {
 *           Result r = ecs.take().get();
 *           if (r != null)
 *               use(r);
 *       }
 *   }
 * 
* * Suppose instead that you would like to use the first non-null result * of the set of tasks, ignoring any that encounter exceptions, * and cancelling all other tasks when the first one is ready: * *
 *   void solve(Executor e,
 *              Collection<Callable<Result>> solvers)
 *     throws InterruptedException {
 *       CompletionService<Result> ecs
 *           = new ExecutorCompletionService<Result>(e);
 *       int n = solvers.size();
 *       List<Future<Result>> futures
 *           = new ArrayList<Future<Result>>(n);
 *       Result result = null;
 *       try {
 *           for (Callable<Result> s : solvers)
 *               futures.add(ecs.submit(s));
 *           for (int i = 0; i < n; ++i) {
 *               try {
 *                   Result r = ecs.take().get();
 *                   if (r != null) {
 *                       result = r;
 *                       break;
 *                   }
 *               } catch (ExecutionException ignore) {}
 *           }
 *       }
 *       finally {
 *           for (Future<Result> f : futures)
 *               f.cancel(true);
 *       }
 *
 *       if (result != null)
 *           use(result);
 *   }
 * 
*/ public class ExecutorCompletionService implements CompletionService { private final Executor executor; private final AbstractExecutorService aes; private final BlockingQueue> completionQueue; /** * FutureTask extension to enqueue upon completion */ private class QueueingFuture extends FutureTask { QueueingFuture(RunnableFuture task) { super(task, null); this.task = task; } protected void done() { completionQueue.add(task); } private final Future task; } private RunnableFuture newTaskFor(Callable task) { if (aes == null) return new FutureTask(task); else return aes.newTaskFor(task); } private RunnableFuture newTaskFor(Runnable task, V result) { if (aes == null) return new FutureTask(task, result); else return aes.newTaskFor(task, result); } /** * Creates an ExecutorCompletionService using the supplied * executor for base task execution and a * {@link LinkedBlockingQueue} as a completion queue. * * @param executor the executor to use * @throws NullPointerException if executor is null */ public ExecutorCompletionService(Executor executor) { if (executor == null) throw new NullPointerException(); this.executor = executor; this.aes = (executor instanceof AbstractExecutorService) ? (AbstractExecutorService) executor : null; this.completionQueue = new LinkedBlockingQueue>(); } /** * Creates an ExecutorCompletionService using the supplied * executor for base task execution and the supplied queue as its * completion queue. * * @param executor the executor to use * @param completionQueue the queue to use as the completion queue * normally one dedicated for use by this service * @throws NullPointerException if executor or completionQueue are null */ public ExecutorCompletionService(Executor executor, BlockingQueue> completionQueue) { if (executor == null || completionQueue == null) throw new NullPointerException(); this.executor = executor; this.aes = (executor instanceof AbstractExecutorService) ? (AbstractExecutorService) executor : null; this.completionQueue = completionQueue; } public Future submit(Callable task) { if (task == null) throw new NullPointerException(); RunnableFuture f = newTaskFor(task); executor.execute(new QueueingFuture(f)); return f; } public Future submit(Runnable task, V result) { if (task == null) throw new NullPointerException(); RunnableFuture f = newTaskFor(task, result); executor.execute(new QueueingFuture(f)); return f; } public Future take() throws InterruptedException { return completionQueue.take(); } public Future poll() { return completionQueue.poll(); } public Future poll(long timeout, TimeUnit unit) throws InterruptedException { return completionQueue.poll(timeout, unit); } }